The disclosure of Japanese Patent Application No. 2000-120698 filed on Apr. 21, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The present invention relates to a throttle control apparatus and method for a direct-fuel-injection-type internal combustion engine that is applied to a direct-fuel-injection-type internal combustion engine in which fuel is directly injected into its cylinders and that controls the degree of opening of a throttle valve for adjusting the amount of air taken into the cylinders.
2. Description of Related Art
Direct-fuel-injection-type internal combustion engines in which fuel is injected directly into the cylinders are known. In this type of internal combustion engine, it is necessary to sufficiently raise the pressure of fuel (fuel pressure) so as to allow fuel injection when the in-cylinder pressure becomes high during the compression stroke. Therefore, in a direct-fuel-injection-type internal combustion engine, a fuel pressure needed for injection is achieved by a mechanical high-pressure fuel pump that is driven by the engine as described in, for example, Japanese Patent Application Laid-Open No. 8-312401.
In such a direct-fuel-injection-type internal combustion engine, however, if the combustion chamber temperature is low at the time of a start of the engine, the fuel injected may deposit on a combustion chamber wall surface. As a result, the amount of fuel that actually contributes to combustion may become insufficient and the state of combustion may deteriorate. Therefore, in order to compensate for such a fuel shortage, the amount of fuel injected is increased. However, at the time of starting the engine initially, the fuel pressure generated by the high-pressure fuel pump is low. As a result, the amount of fuel that can be injected is correspondingly limited, as described in the aforementioned literature. Therefore, at the time of starting the engine initially, the air-fuel ratio of mixture around ignition plugs is on the fuel-lean side and misfires occur, thereby impeding stable operation of the engine, for example, fluctuating engine revolution, or the like.
It is an object of the invention to provide a throttle control apparatus and method for a direct-fuel-injection-type internal combustion engine that is capable of operating the engine with high stability.
In accordance with a first aspect of the invention, a throttle control apparatus for a direct-fuel-injection-type internal combustion engine in which fuel is injected directly into a cylinder includes: a throttle valve for adjusting an amount of intake air drawn into the cylinder; and a controller that, when the engine is to be started, sets the throttle valve to a degree of opening that is on a closed valve side of a post-engine start target degree of opening. Further, it is preferable that when the engine is to be started, the controller may sets the throttle valve to a closed valve state, and when it is determined that a start of the engine has been accomplished, the controller may open the throttle valve by gradually increasing the degree of opening of the throttle valve from the degree of opening of the closed valve state to the post-engine start target degree of opening.
Therefore, at the time of starting the engine initially, when the temperature in the cylinder is low, the degree of opening of the throttle valve is set to a degree of opening that is on the closed valve side of the post-engine start target degree of opening, so that the throttle valve is set to the closed valve state and the pressure in the cylinder is kept low. As a result, the pressure difference between the fuel injection pressure and the pressure occurring in the cylinder becomes great, so that injected fuel is well atomized and the spraying of fuel is accelerated. Hence, the amount of fuel that does not spray but deposits on inner surfaces of the combustion chamber reduces, and the amount of fuel that actually contributes to combustion increases. Consequently, even in a situation where the amount of fuel injected cannot be sufficiently increased, it is possible to avoid the occurrence of a fuel-lean mixture around the ignition plug and therefore avoid occurrence of a misfire.
However, if the throttle valve is set to the closed valve state at the time of starting the engine initially, the degree of opening of the throttle valve, after the start of the engine has been accomplished, is changed from the degree of opening of the closed valve state to the post-engine start target degree of opening. If in this case, the degree of opening of the throttle valve is controlled simply in accordance with changes of the target degree of opening, the amount of intake air will temporarily increase so that the spraying of injected fuel will deteriorate. This incurs a danger of deterioration of the combustion state and therefore a danger of engine revolution fluctuations. According to the above-described construction, however, after it is determined that the start of the engine has been accomplished, the throttle valve is opened in such a manner that the degree of opening thereof is gradually increased from the degree of opening of the closed valve state to the post-engine start target degree of opening. Therefore, sharp changes in the amount of intake air are curbed, and fluctuations of engine revolution are reduced. Hence, this construction makes it possible to achieve stable operation of the direct-fuel-injection-type internal combustion engine at the time of starting the engine initially.
In the above-described aspect, the controller may determine that a start of the engine has been accomplished, on a condition that an engine revolution speed has exceeded a predetermined revolution speed.
According to this construction, the engine revolution speed is employed to appropriately determine that the start of the engine has been accomplished. This makes it possible to prevent the throttle valve from being held in the closed valve state longer than necessary, and makes it possible to appropriately increase the engine revolution speed.
Furthermore, in the above-described aspect, a predetermined revolution speed may be variably set in accordance with an engine temperature of the internal combustion engine.
The readiness of the spraying of injected fuel changes in accordance with the engine temperature. For example, the spraying of injected fuel deteriorates more greatly if the engine temperature is lower and the temperature of the cylinder peripheral wall and the piston top surface on which spray of fuel impinges is lower. Therefore, if the predetermined revolution speed is set to a relatively low revolution speed, an undesired event described below is likely, for example, when the engine temperature is very low. That is, it is likely that although the starting of the engine has not been appropriately accomplished, it will be determined that the start of the engine has been accomplished. In such a case, the amount of intake air will be increased so that the spraying of injected fuel will deteriorate, thus leading to a misfire. However, according to the above-described construction, the engine revolution speed criterion (the predetermined engine revolution speed), which is employed for the determination as to whether the start of the engine has been accomplished, is variably set in accordance with the engine temperature. Therefore, it becomes possible to properly determine that the start of the engine has been accomplished and to properly increase the engine revolution speed, independently of whether the engine temperature is high or low.
Still further, in the above-described aspect, the controller may gradually increase the degree of opening of the throttle valve from the degree of opening of the closed valve state to the post-engine start target degree of opening, after a predetermined delay time elapses and after it is determined that the start of the engine has been accomplished.
In multi-cylinder internal combustion engines, individual cylinders sequentially undergo the explosion stroke at intervals. In some cases, therefore, immediately after it is determined that the start of the engine has been accomplished, the combustion chamber temperature is yet to be sufficiently raised by combustion in one or more cylinders. Therefore, if the degree of opening of the throttle valve is increased to the post-engine start target degree of opening immediately after it is determined that the start of the engine has been accomplished, an undesired event may occur in which the amount of intake air is increased although the combustion chamber temperature has not been sufficiently raised in one or more cylinders. In such a case, the spraying of fuel may deteriorate and the combustion state may deteriorate.
According to the above-described construction, however, the throttle valve is opened after the elapse of a predetermined delay time following the determination that the start of the engine has been accomplished. This ensures that the amount of intake air will be increased after the temperature of the combustion chamber of each cylinder is raised without fail. Hence, the deterioration of the combustion state as mentioned above can be reduced.
Furthermore, in the above-described aspect of the invention, the controller, after a start of the engine has been accomplished, may open the throttle valve by gradually increasing the target degree of opening of the throttle valve in accordance with transition of at least one of an intake pipe negative pressure, an amount of intake air, and an engine revolution speed.
According to this construction, when the start of the engine has been accomplished, the throttle valve can be opened to the post-engine start target degree of opening in a manner that is more suitable to the normal engine operation state. Therefore, it becomes possible to further stabilize the operation of the direct-fuel-injection-type internal combustion engine at the time of starting the engine initially.
Another aspect of the invention involves a method of controlling the throttle valve.